Supplemental inflatable restraint (SIR) systems for automotive vehicles generally employ a firing circuit having a squib for causing inflation of an air bag, and a deployment circuit having an accelerometer sensitive to vehicle motion, especially deceleration, and a microprocessor monitoring the accelerometer output for evaluating the severity of a crash to determine whether to deploy the air bag. The vehicle battery or ignition system voltage empowers the deployment circuit and the firing circuit. The deployment circuit and the firing circuit are to a large extent embodied with a microprocessor on an integrated circuit chip, but an external harness leads to the squib at the site of the air bag, or in the event the vehicle is equipped with two or more air bags, the harness connects the squib for each air bag to the chip.
It is conceivable that some portion of the harness might become damaged and short to ground or even to the vehicle battery voltage. To forestall the possibility that such inadvertent electrical connection to the firing circuit might cause deployment of the air bag or disable the system, it is desirable to monitor the system to detect any such event. It is already known to diagnose a short of the firing circuit by feeding a small test current through the squib and through a biasing resistor to ground. The resulting bias voltage will depend on the current and if there are no shorts to the firing circuit the resulting test voltage will be at a prescribed value. To assure this, the test current is carefully controlled. Where, for example, the current is provided by a current source on an integrated circuit, the circuit must be trimmed during manufacture to assure the correct current output. The test voltage is sampled by an A/D converter and fed to the microprocessor where it is monitored to detect a low value indicative of shorting to ground or a high value indicative of shorting to supply voltage.
Since the number of inflatable restraints per vehicle varies according to vehicle design, it is desirable to provide a SIR system to accommodate any number of restraints expected to be employed. This has been accomplished previously by designing the diagnostic circuit for the maximum number of air bags--that is, tailoring the current sources and the bias resistor to give a desired test voltage. Then if a given vehicle has fewer than the maximum number, the SIR harness is modified to couple current sources across the connections where the "missing" squibs would have been connected, to properly bias the resistor to the desired test voltage. This arrangement is not desirable from the vehicle manufacturers point of view, resulting in costs which might be avoided; also the need to trim the current sources increases system cost.